1. Field of the Invention
The present invention relates to a liquid crystal display.
2. Description of the Related Art
An active-matrix-type liquid crystal display such as a TFT (Thin Film Transistor) liquid crystal panel is expected to be widely used as a display of a television for home use or an OA apparatus. This is because, according to the active-matrix-type liquid crystal display, it is possible to easily obtain thin and light-weight display device, and, also, to provide image display which has a quality not lower than that of a conventional CRT display.
Furthermore, as this type of display device is thin and of a light weight, it is expected to employ it not only for a portable information apparatus such as a notebook-type personal computer, but also other various multimedia information apparatus. For this purpose, development for a liquid crystal display employing a block sequential driving scheme, by which it is possible to obtain a thin and light-weight liquid crystal display of an effectively reduced frame size, a high definition, and, also, a large-sized screen, has been proceeded.
A panel structure of the active-matrix-type liquid crystal display, which is a type of a flat-panel display, and can provide a high-quality image display, will now be described. The active-matrix-type liquid crystal display has a panel structure such that, as shown in FIG. 1, liquid crystal is sealed between a TFT substrate which has pixel electrodes 110 disposed in a matrix manner and switching devices (TFT, etc.) 112 prepared for the respective pixel electrodes 110, and a common substrate on which common electrodes are formed throughout the surface thereof.
Data signal lines DL and scanning lines (scanning electrodes) 114 intersect by a matrix manner on the TFT substrate, and a TFT is connected to each of all the intersections as the switching device 112. The TFTs on the line selected by the scanning line 114 are turned on, an image signal voltage applied to the data signal line DL is applied to each pixel electrode 110, and the information is held until the line is selected subsequently, as a result of the electric charge being held there.
Since inclination of the liquid crystal molecule is determined according to the thus-held information, the amount of light transmission by the liquid crystal display panel can be controlled thereby, and, thus, expression of various gray scales can be attained there. Furthermore, for a color display, mixture of light is performed by using an RGB color filter. as well-known.
FIGS. 2A and 2B illustrate configurations of liquid crystal displays in the related art. In the liquid crystal display shown in FIG. 2A, a scanning line driver 1, a top data signal line driver 3 and a bottom data signal line driver 5 to each of which data for display is supplied, are arranged around a display area 100. In the liquid crystal display shown in FIG. 2B, a data signal line driver 7 is provided only on one side of the display area 100.
Thus, for general liquid crystal displays in the related art, respective driver ICs for data signal lines and scanning lines are arranged around the panel and mounted by TAB press-fixing way or a COG (Chip On Glass) mounting way, and, thereby, each bus line is driven, and, thus, image display is performed through the display area 100.
Recently, development of liquid crystal displays of polysilicon LCD type in which a driving circuit is formed directly on a glass substrate (TFT substrate of the liquid crystal panel) by using polysilicon material. According to the liquid crystal display of this polysilicon LCD type, a driving circuit is formed on the glass substrate, and, thereby, a so-called frame space provided around the display screen or the like can be effectively eliminated. Further, since the circuit structure is also formed during a TFT substrate manufacturing process, a separate process of assembling IC after that can be eliminated. Thereby, the polysilicon LCD type liquid crystal display is further demanded to have a larger-sized screen and a higher display definition.
For such a driving circuit of the liquid crystal panel of the above-mentioned polysilicon LCD type, study has been proceeded for a scheme in which display data is sent to respective blocks of the display area through data signal lines sequentially (block sequential driving scheme), and thus, the panel is driven for respective divisions thereof sequentially.
FIG. 3 shows a configuration of a liquid crystal display employing the above-mentioned block sequential driving scheme in the related art. This liquid crystal display includes a display area 100, a driver IC 9, a shift register 11, a multiplexer 13, a buffer 15, and video lines VL and analog switches AS. In this configuration, the number of pixels of the display area 100 is set as (800×RGB×600), also, it is divided into blocks BL1 through BL8. To the driver IC 9, eight block selection pulses SBL1 through SBL8, display signals D1 through D300, and a gate control signal GC are input.
Further, the analog switches AS are turned on according to the block selection pulses, and therethrough, the display signals are supplied to the corresponding blocks via the video lines VL and the data signal lines DL.
In this configuration, as the circuit formed on a glass substrate is merely a selector part in which the analog switches AS are disposed side by side, it is possible to effectively simplify the circuit included in the liquid crystal display, and, thus, it is possible to improve the yield of the liquid crystal panels.
Furthermore, since a conventional general-purpose data driver can be utilized there, it becomes possible to produce the polysilicon LCD at a low cost. Further, development for realization of a high-display-definition polysilicon LCD having a large-sized screen has been further proceeded by increase in the number of divided blocks, employing a multi-output IC of a general-purpose data driver, etc.
However, in case a general-purpose driver is used there, improvement in the driving capability of the driver is demanded. Namely, in the conventional driver, the required charging-up time for the data bus and video lines for the respective blocks may not match the timing of driving for the respective divided blocks. In such a situation, switching of data may not be performed smoothly, and, may result in problematic image display.
In the liquid crystal panel formed through an amorphous silicon process, only pixel cells arranged in a matrix manner in the display area, switching devices, data signal lines, and scanning lines are provided. The pixel cells include pixel electrodes 110, common electrodes opposite thereto, and liquid crystal layers provided therebetween.
Further, the data driver which performs display drive of this panel has an analog or multi-bit digital gray scale signal input thereto from a personal computer, etc., and, in case of the digital gray scale signal, a multi-gray scale is displayed by converting it into an analog gray scale voltage of the 64 gray scales or the 256 gray scales which is then provided to the liquid crystal panel.
In the liquid crystal panel (may be simply referred to as a “panel”) formed using polysilicon, all or part of the driving circuit may be formed on the panel glass together with the display area, as a peripheral circuit of the display area. Then, this driving circuit may be used as the scanning line driver. Also, part of the driving circuit may be used as the data signal line driver, is formed on the panel glass, and this data signal line peripheral circuit is controlled by a control circuit outside of the panel. Thereby, the above-mentioned block sequential driving scheme can be realized.
However, in case of the peripheral circuit provided on the panel formed using polysilicon, by a reason concerning the characteristic of transistors included in the peripheral circuit, not an IC logic level such as 3.3 V but a higher voltage such as 10 V or higher, for example, is needed for driving the peripheral circuit. Therefore, level conversion is needed between the logic level of the control signal used in the outside of the panel and the operation voltage of the peripheral circuit formed on the panel.